1. Slow down of the THC and increasing hurricane activity
Two articles in Nature and the role of multidecadal variability in the Atlantic sector
Emanuel, 2005: Increasing hurricane destructiveness
Bryden et al., 2005: Slow down of the THC by 30%
What we see is mostly multidecadal variability and the response to global warming is tricky (Latif et al., 2006; Latif, Keenlyside, Bader, 2006)

2. Multidecadal variability in the North Atlantic region

3. „The day after tomorrow“

4. „Alarming“ MOC change at 25°N
Slowing (30%) of the Atlantic meridional overturning circulation at 25°N, Bryden et al. (2005), Nature*).
The Atlantic heat conveyor slows, Quadfasel (2005), Nature.
The Atlantic conveyor may have slowed, but don´t panic, Kerr (2005), Science.
*)northward heat transport ca PW (1957, 1981, 1992)
northward heat transport ca. 1.1 PW (1998, 2004)
Is the MOC weakening already underway?

5. Assesment of density changes in the „overflows“ using ocean model experiments
Δρobs ~ 1Sv
models
change in MOC (Sv)
observed
change in overflow density (kg/m3)

6. Observed SST trend 1980-2004, global mean subtracted
MOC fingerprint: ΔSST

7. The SST dipole index, a measure for the MOC strength
The multidecadal variability is much larger than the linear trend. The variations in the dipole index correspond to about ±1.5-3 Sv (MPI-model).

8. Ocean model simulations support the MOC strenghtening
full forcing
add. realizations
heat flux only
anomaly relative to control run (Sv)
ORCA-LIM, ~0.5°
 (45°N)
time (years)

9. The dipole index and the NAO (low-pass filterted)
The dipole index (MOC) follows the NAO with a time lag of about 10 years
SST dipole
NAO

10. The new IPCC (4AR) simulations
„best estimate“ -25% until 2100 (-43Sv)
„best estimate“

11. Conclusions I
The SST of the Atlantic may serve as a fingerprint to detect MOC changes.
If so, a significant weakening of the MOC cannot be detected against the strong multidecadal variability.
The multidecadal MOC variability can be understood as the response to the NAO.
The latest IPCC models simulate only a moderate weakening of the MOC, with a best estimate of about 25% reduction until 2100.
An anthropogenic weakening of the MOC is unlikely to leave the range of the internal variability during the next few decades.

12. Hurricane Katrina

13. The ACE Index, a measure of kinetic energy (~v2)
2005

14. Trends in tropical sea surface temperature
Trop. NA
Indo-Pacific

15. The role of the vertical wind shear (ECHAM5-T106)

16. ACE Index, vertical wind shear, and SST difference tropical North Atlantic/Indo-Pacific
r(ACE/SSTdiff.)=

17. Conclusions II
Tropical Atlantic SST exhibits a strong trend over the last century.
Hurricane activity in the Atlantic sector does not exhibit a sustained long-term trend.
This is due to the competing effects of the tropical oceans on the vertical wind shear over the Atlantic.
The SST difference between the tropical North Atlantic and the Indo-Pacific correlates well with the hurricane activity.
Thus, the Atlantic needs to warm more than the Indo-Pacific for the hurricane activity to increase.